This invention relates to a process for removing sulfur-containing impurities from olefin-containing hydrocarbon mixtures. More particularly, the process involves converting the feedstock to an intermediate product of reduced bromine number, separating the intermediate product into fractions of different boiling point, and subjecting the low boiling fraction to hydrodesulfurization.
The fluidized catalytic cracking process is one of the major refining processes which is currently employed in the conversion of petroleum to desirable fuels such as gasoline and diesel fuel. In this process, a high molecular weight hydrocarbon feedstock is converted to lower molecular weight products through contact with hot, finely-divided, solid catalyst particles in a fluidized or dispersed state. Suitable hydrocarbon feedstocks typically boil within the range from about 205xc2x0 C. to about 650xc2x0 C., and they are usually contacted with the catalyst at temperatures in the range from about 450xc2x0 C. to about 650xc2x0 C. Suitable feedstocks include various mineral oil fractions such as light gas oils, heavy gas oils, wide-cut gas oils, vacuum gas oils, kerosenes, decanted oils, residual fractions, reduced crude oils and cycle oils which are derived from any of these as well as fractions derived from shale oils, tar sands processing, and coal liquefaction. Products from a fluidized catalytic cracking process are typically based on boiling point and include light naphtha (boiling between about 10xc2x0 C. and about 221xc2x0 C.), heavy naphtha (boiling between about 10xc2x0 C. and about 249xc2x0 C.), kerosene (boiling between about 180xc2x0 C. and about 300xc2x0 C.), light cycle oil (boiling between about 221xc2x0 C. and about 345xc2x0 C.), and heavy cycle oil (boiling at temperatures higher than about 345xc2x0 C.).
Naphtha from a catalytic cracking process comprises a complex blend of hydrocarbons which includes paraffins (also known as alkanes), cycloparaffins (also known as cycloalkanes or naphthenes), olefins (as used herein, the term olefin includes all acyclic and cyclic hydrocarbons which contain at least one double bond and are not aromatic), and aromatic compounds. Such a material typically contains a relatively high olefin content and includes significant amounts of sulfur-containing aromatic compounds, such as thiophenic and benzothiophenic compounds, as impurities. For example, a light naphtha from the fluidized catalytic cracking of a petroleum derived gas oil can contain up to about 60 wt. % of olefins and up to about 0.7 wt. % of sulfur wherein most of the sulfur will be in the form of thiophenic and benzothiophenic compounds. However, a typical naphtha from the catalytic cracking process will usually contain from about 5 wt. % to about 40 wt. % olefins and from about 0.07 wt. % to about 0.5 wt. % sulfur.
Not only does the fluidized catalytic cracking process provide a significant part of the gasoline pool in the United States, it also provides a large proportion of the sulfur that appears in this pool. The sulfur in the liquid products from this process is in the form of organic sulfur compounds and is an undesirable impurity which is converted to sulfur oxides when these products are utilized as a fuel. The sulfur oxides are objectionable air pollutants. In addition, they can deactivate many of the catalysts that have been developed for the catalytic converters which are used on automobiles to catalyze the conversion of harmful engine exhaust emissions to gases which are less objectionable. Accordingly, it is desirable to reduce the sulfur content of catalytic cracking products to the lowest possible levels.
Low sulfur products are conventionally obtained from the catalytic cracking process by hydrotreating either the feedstock to the process or the products from the process. The hydrotreating process involves treatment of the feedstock with hydrogen in the presence of a catalyst and results in the conversion of the sulfur in the sulfur-containing impurities to hydrogen sulfide, which can be separated and converted to elemental sulfur. The hydrotreating process can result in the destruction of olefins in the feedstock by converting them to saturated hydrocarbons through hydrogenation. This destruction of olefins by hydrogenation is usually undesirable because: (1) it results in the consumption of expensive hydrogen, and (2) the olefins are usually valuable as high octane components of gasoline. As an example, a typical naphtha of gasoline boiling range from a catalytic cracking process has a relatively high octane number as a result of a large olefin content. Hydrotreating such a material causes a reduction in the olefin content in addition to the desired desulfurization, and the octane number of the hydrotreated product decreases as the degree or severity of the desulfurization increases.
U.S. Pa. No. 5,865,988 (Collins et al.) is directed to a two step process for the production of low sulfur gasoline from an olefinic, cracked, sulfur-containing naphtha. The process involves: (a) passing the naphtha over a shape selective acidic catalyst, such as ZSM-5 zeolite, to selectively crack low octane paraffins and to convert some of the olefins and naphthenes to aromatics and aromatic side chains; and (2) hydrodesulfurizing the resulting product over a hydrotreating catalyst in the presence of hydrogen. It is disclosed that the initial treatment with the shape selective acidic catalyst removes the olefins which would otherwise be saturated in the hydrodesulfurization step.
International Patent Application No. WO 98/30655 (Huff et al.), published under the Patent Cooperation Treaty, discloses a process for the production of a product of reduced sulfur content from a feedstock wherein the feedstock is comprised of a mixture of hydrocarbons and contains organic sulfur compounds as unwanted impurities. This process involves converting at least a portion of the sulfur-containing impurities to sulfur-containing products of a higher boiling point by treatment with an alkylating agent in the presence of an acid catalyst and removing at least a portion of these higher boiling products by fractionation on the basis of boiling point.
U.S. Pat. Nos. 5,298,150 (Fletcher et al.); 5,346,609 (Fletcher et al.); 5,391,288 (Collins et al.); and 5,409,596 (Fletcher et al.) are all directed to a two step process for the preparation of a low sulfur gasoline wherein a naphtha feedstock is subjected to hydrodesulfurization followed by treatment with a shape selective catalyst to restore the octane which is lost during the hydrodesulfurization step.
U.S. Pat. No. 5,171,916 (Le et al.) is directed to a process for upgrading a light cycle oil by: (1) alkylating the heteroatom containing aromatics of the cycle oil with an aliphatic hydrocarbon having at least one olefinic double bond through the use of a crystalline metallosilicate catalyst; and (2) separating the high boiling alkylation product by fractional distillation. It is disclosed that the unconverted light cycle oil has a reduced sulfur and nitrogen content, and the high boiling alkylation product is useful as a synthetic alkylated aromatic functional fluid base stock.
U.S. Pat. No. 5,599,441 (Collins et al.) discloses a process for removing thiophenic sulfur compounds from a cracked naphtha by: (1) contacting the naphtha with an acid catalyst in an alkylation zone to alkylate the thiophenic compounds using the olefins present in the naphtha as an alkylating agent; (2) removing an effluent stream from the alkylation zone; and (3) separating the alkylated thiophenic compounds from the alkylation zone effluent stream by fractional distillation. It is also disclosed that the sulfur-rich high boiling fraction from the fractional distillation may be desulfurized using conventional hydrotreating or other desulfurization processes.
U.S. Pat. No. 5,863,419 (Huff, Jr. et al.) discloses a catalytic distillation process for the production of a product of reduced sulfur content from a feedstock wherein the feedstock is comprised of a mixture of hydrocarbons which contains organic sulfur compounds as unwanted impurities. The process involves carrying out the following process steps simultaneously within a distillation column reactor: (1) converting at least a portion of the sulfur-containing impurities to sulfur-containing products of a higher boiling point by treatment with an alkylating agent in the presence of an acid catalyst; and (2) removing at least a portion of these higher boiling products by fractional distillation. It is also disclosed that the sulfur-rich high boiling fraction can be efficiently hydrotreated at relatively low cost because of its reduced volume relative to that of the original feedstock.
Hydrocarbon liquids which boil at standard pressure over either a broad or a narrow range of temperatures within the range from about 10xc2x0 C. to about 345xc2x0 C. are referred to herein as xe2x80x9chydrocarbon liquids.xe2x80x9d Such liquids are frequently encountered in the refining of petroleum and also in the refining of products from coal liquefaction and the processing of oil shale or tar sands, and these liquids are typically comprised of a complex mixture of hydrocarbons, and these mixtures can include paraffins, cycloparaffins, olefins and aromatics. For example, light naphtha, heavy naphtha, gasoline, kerosene and light cycle oil are all hydrocarbon liquids.
Hydrocarbon liquids which are encountered in a refinery frequently contain undesirable sulfur-containing impurities which must be at least partially removed. Hydrotreating procedures are effective and are commonly used for removing sulfur-containing impurities from hydrocarbon liquids. Unfortunately, conventional hydrotreating processes are usually unsatisfactory for use with highly olefinic hydrocarbon liquids because such processes result in significant conversion of the olefins to paraffins which are usually of lower octane. In addition, the hydrogenation of the olefins results in the consumption of expensive hydrogen.
Organic sulfur compounds can also be removed from hydrocarbon liquids by a multiple step process which comprises: (1) conversion of the sulfur compounds to products of higher boiling point by alkylation; and (2) removal of the higher boiling products by fractional distillation. Such a process is relatively inexpensive to carry out, and it does not usually result in any significant octane loss. Although this type of process is quite effective in removing a large portion of aromatic, sulfur-containing, organic impurities, such as thiophenic and benzothiophenic compounds, the product from such a process will typically contain a much reduced but still significant sulfur content. In addition, such a process is frequently not very satisfactory in removing other common types of sulfur containing impurities, such as mercaptans.
Accordingly, there is a need for a process which can achieve a substantially complete removal of sulfur-containing impurities from olefin-containing hydrocarbon liquids which: (1) is inexpensive to carry out, and (2) results in little if any octane loss. For example, there is a need for such a process which can be used to remove sulfur-containing impurities from hydrocarbon liquids, such as products from a fluidized catalytic cracking process, which are highly olefinic and contain relatively large amounts of sulfur-containing organic materials such as mercaptans, thiophenic compounds, and benzothiophenic compounds as unwanted impurities.
We have discovered such an improved process which involves modifying the olefin content of the feedstock over an olefin-modification catalyst in an olefin-modification step, fractionating the products from the olefin-modification step into at least two fractions on the basis of boiling point, and hydrodesulfurizing at least the lowest boiling of the resulting fractions. The olefin-modification step results in a reduction of the olefinic unsaturation of the feedstock, as measured by bromine number. As a consequence of the olefin-modification step, a product is obtained from the subsequent hydrodesulfurization step which has little loss of octane relative to that of the feedstock to the olefin-modification step. In addition, the reduction of olefinic unsaturation in the olefin-modification step results in a corresponding reduction of hydrogen consumption in the hydrodesulfurization step since there is a reduced number of olefinic double bonds to consume hydrogen in hydrogenation reactions.
One embodiment of the invention is a process for producing a product of reduced sulfur content from a feedstock, wherein said feedstock contains sulfur-containing organic impurities and is comprised of a normally liquid mixture of hydrocarbons which includes olefins, said process comprising:
(a) contacting the feedstock with an olefin-modification catalyst in an olefin-modification reaction zone under conditions which are effective to produce a product having a bromine number which is lower than that of the feedstock;
(b) fractionating the product from said olefin-modification reaction zone to produce:
(i) a first fraction which comprises sulfur-containing organic impurities and has a distillation endpoint which is in the range from about 135xc2x0 C. to about 221xc2x0 C.; and
(ii) a second fraction which is higher boiling than the first fraction and comprises sulfur-containing organic impurities; and
(c) contacting said first fraction with a hydrodesulfurization catalyst in the presence of hydrogen in a first hydrodesulfurization reaction zone under conditions which are effective to convert at least a portion of the sulfur in said sulfur-containing impurities of the first fraction to hydrogen sulfide.
Another embodiment of the invention is a process for producing products of reduced sulfur content from a feedstock, wherein said feedstock contains sulfur-containing organic impurities and is comprised of a normally liquid mixture of hydrocarbons which includes olefins, said process comprising:
(a) contacting the feedstock with an olefin-modification catalyst in an olefin-modification reaction zone under conditions which are effective to produce a product which has a lower bromine number than that of the feedstock, wherein said olefin-modification catalyst is selected from the group consisting of solid phosphoric acid catalysts and acidic polymeric resin catalysts;
(b) fractionating the product from said olefin-modification reaction zone to produce:
(i) a first fraction which contains sulfur-containing organic impurities and has a distillation endpoint which is in the range from about 135xc2x0 C. to about 221xc2x0 C.; and
(ii) a second fraction which is higher boiling than the first fraction and contains sulfur-containing organic impurities; and
(c) contacting said first fraction with a hydrodesulfurization catalyst in the presence of hydrogen in a first hydrodesulfurization reaction zone under conditions which are effective to convert at least a portion of the sulfur in said sulfur-containing impurities of the first fraction to hydrogen sulfide.
An object of the invention is to provide an improved process for the removal of sulfur-containing impurities from a hydrocarbon liquid which contains a significant olefin content.
Another object of the invention is to provide an improved method for the efficient removal of sulfur-containing impurities from an olefinic cracked naphtha.
A further object of the invention is to provide an improved method for desulfurizing an olefinic cracked naphtha which yields a product of substantially unchanged octane.